A mass excavation contractor reported that an XCMG XE370CA 37-ton excavator's bucket cylinder would bounce slightly when curling an empty bucket, and the machine felt underpowered when attempting to dig into hard clay. The boom and arm functions also felt inconsistent-sometimes fast, sometimes sluggish, without an obvious pattern. The engine RPM was stable, indicating no drivetrain or engine issues.
The technician first recognized the pattern of a load-sensing control problem. The XE370CA uses a "Load-Sensing (LS) Main Control Valve" system. Each control valve section includes an "LS Shuttle Valve" that detects the highest load pressure across all active functions and routes it back to the main pump regulator as a reference signal. The regulator then adjusts the pump displacement to maintain a constant 25-bar pressure margin above the highest load pressure-delivering just enough flow to satisfy the demand. When the LS signal is incorrect, the pump either over-delivers flow (causing erratic or "bouncy" cylinder response) or under-delivers (causing slow functions under load).
He installed a pressure gauge on the "LS Signal Line" at the main pump regulator. With the engine at idle and the joysticks in neutral, the LS pressure should read 25 bar below the pump's standby margin (typically around 35 bar absolute). The gauge read only 12 bar-a low, inconsistent signal. He then tested the LS signal at each individual control valve section's shuttle valve port. At the "Bucket Control Valve" section, the LS signal read 12 bar. At the boom and arm sections, the LS signal read a normal 35 bar. This isolated the fault to the bucket section's shuttle valve.
He removed the bucket section's "LS Shuttle Valve" cartridge. The internal shuttle poppet was stuck in a partially open position, held there by a small, flattened piece of sealing material that had migrated from an upstream fitting's O-ring. Because the shuttle poppet could not seat fully, the bucket circuit's load pressure was bleeding into the LS signal line, dragging the entire LS signal down to 12 bar regardless of what function the operator was demanding. The pump regulator interpreted this low LS signal as a low-demand condition and destroked the pump. During a light load (empty bucket curl), the low pump flow produced the "bouncy" response. During a heavy load (hard clay digging), the insufficient pump flow produced the sluggish response.
The repair involved extracting and cleaning the bucket section's LS shuttle valve cartridge, removing the migrated O-ring debris, and verifying the shuttle poppet moved freely through its full stroke. The hydraulic system was flushed to remove any remaining debris, and the source of the debris was traced to a degraded O-ring on the bucket cylinder's hose fitting-replaced as part of the repair. Fresh hydraulic oil and filters were installed. After reassembly, the LS signal read 35 bar at idle, and the bucket cylinder curled smoothly without bounce under no-load and maintained full force when digging into compacted clay.
